Power Chord are usually found in rock and metal music.  They are extremely simple if you can do bar chords, (which is cover in another lesson.)  Power chords use the three lower notes of your chord whereas regular chords feed off of the higher strings as well.  all right, now it's time for your first power chord, E5, As you see, it's the same fingering as Em... only you don't hit all the strings.  Why E5 instead of E?  This simple formula: root/fifth/octave

So now what if I want's to do A5?  You may wonder, simple, you bar the Em fingering on the 5'th fret like so.  Then you hit the three low strings.

The same thing for B5, just on the 7'th fret.

Also, the power chord is moveable and can be played in several places on the neck as you can see from the B5 below, (the same fingering of the chord on the open top  is A.)

Power chords are often overlooked because of their easiness, but hey, it sounds cool, and heavy and all, and in my opinion if it sounds good, more power to 'ya.  I'll close this with a powerchord riff by MxPx ("Party My House Be There")

E5-F#5-A5-B5 2x's  A5-G#5-B5

(Click Here For An MP3 example)